原来的需求是这样的，用户生成了400元订单，需要400元的发票去抵消，发票数和订单数不受限制，

将问题拟物化，订单可以想象成杯子，发票可以想象成水，现在要把水分配到每个杯子中去

我们先记录每个杯子的起始值和结束值，还有当前值，起始值（start）=当前杯子的起始值加上上一个杯子的结束值，结束值（end）即起始值加上杯子的容量，当前值（index）即当前杯子中的水，如果大于杯子的容量则倒下一个杯子，依次循环，所以其实整个倒水可以看做一个大杯子，按阶段倒水，每次记录倒水的index（即当前（或者当次）杯子中的水即可）

现在假设a杯50L， b杯20L

假如第一次倒， a[0, 50]杯子倒满，剩余20；index = 50 , start = 0 , end = 50;第二次倒b杯[50, 70]， 首先拿上一下剩余的20L， 刚好倒满，index = 20; start = 50 ,end = 70依次类推

话不多说，上代码

export function getItemDistribution(a, b, waterKey, cupKey, policyId) {    class ItemDistribution {         cupList = [];        result = [];        constructor(list, maps) {            this.waterKey = waterKey;            this.cupKey = cupKey;            this.policyId = policyId;            this.sort(list, maps);            this.init(list);        }        sort(list, maps) {            let {waterKey, cupKey} = this;            if (waterKey) {                list = list.sort((a, b) => a[waterKey] - b[waterKey]);            } else {                list = list.sort((a, b) => a - b);            }            if (cupKey) {                maps = maps.sort((a, b) => a[cupKey] - b[cupKey]);            } else {                maps = maps.sort((a, b) => a - b);            }            this.list = list;            this.maps = maps;            return {list, maps}        }        init(list) {            if (list == null || list.length <= 0) {                return;            }            let start = 0, end = 0, index = 0;            let {waterKey} = this;            list.forEach((item, id) => {                if (!waterKey) {                    this.result[id] = [];                }                let water = waterKey ? item[waterKey] : item;                end = start + Number(water);                this.entry({start, index, end});                start = end;            });            this.dispatchMaps();        }        entry({start, index, end}) {            index = start;            this.cupList.push({start, end, index})        }        dispatchMaps() {            function isEmpty(o) {                switch (JSON.stringify(o)) {                    case 'null':                    case undefined:                        return true;                    case '[]':                        return true;                }                return false;            }            let {maps = [], cupList, cupKey} = this;            if (isEmpty(cupList) || isEmpty(maps)) return false;            maps.forEach((item, id) => {                let cup = cupKey ? item[cupKey] : item;                this.dispatchCup({item: cup, id});            });        }        getCup() {            function isEqual(a, b) {                return String(a.toFixed(2)) !== String(b.toFixed(2));            }            let {cupList} = this;            for (let i = 0; i < cupList.length; i++) {                let item = cupList[i];                if (isEqual(item.end, item.index)) {                    return {cup: item, cupIndex: i};                }            }            return {};        }        dispatchCup({item, id}) {            let {cup, cupIndex} = this.getCup();            if (!cup) return false;            let {index, end} = cup;            let remain = end - index;            let {waterKey, list, policyId, maps} = this;            if (remain > item) {                cup.index = cup.index + item;                waterKey ? this.result.push({                    ...list[cupIndex],                    [waterKey]: item,                    [policyId]: maps[id][policyId]                }) : this.result[id].push(item);                return false;            } else {                cup.index = cup.end;                waterKey ? this.result.push({                    ...list[cupIndex],                    [waterKey]: remain,                    [policyId]: maps[id][policyId]                }) : this.result[id].push(remain);                item -= remain;                this.dispatchCup({item, id})            }        }    }    return new ItemDistribution(a, b);}